The IBM System/360 is a solid-state, program com­
patible, data processing system providing the speed,
precision, and data manipulating versatility demand­
ed by the challenge of commerce, science, and in­
dustry. System/360, with advanced logical design im­
plemented by microminiature technology, provides a
new dimension of performance, flexibility, and relia­
bility. This dimension makes possible a new, more
efficient systems approach to all areas of information
processing, with economy of implementation and ease
of use. System/360 is a single, coordinated set of new
data processing components intended to replace the
old logical structure with an advanced creative de­
sign for present and future application.
The logical design of System/360 permits efficient
use at several levels of performance with the preser­
vation of upward and downward program compati­
bility. Extremely high performance and reliability re­
quirements are met by combining several models into
one multisystem using the multisystem feature.
General-Purpose Design System/360 is a general-purpose system designed to
be tailored for commercial, scientific, communications,
or control applications. A Standard instruction set pro­
vides the basic computing function of the system .. To
this set a decimal feature may be added to provide a
Commercial instruction set or a floating-point feature
may be added to provide a Scientific instruction set.
When the storage protection feature is added to the
commercial and scientific features, a Universal set is
obtained. Direct control and timer features may be
added to satisfy requirements for TELE-PROCESSINC@ systems to allow load-sharing or to satisfy real-time
needs. System/360 can accommodate large quantities of
addressable storage. The markedly increased capaci­
ties over other present storage is provided by the
combined use of high-speed storage of medium size
and large-capacity storage of medium speed. Thus,
the requirements for both performance and size are
satisfied in one system by incorporating a heirarchy
of storage units. The design also anticipates future
development of even greater storage capacities. Sys­ tem/360 incorporates a standard method for attaching
input/output devices differing in function, data rate, IBM System/360
and access time. An individual System/360 is obtained
by selecting the system components most suited to the
applications from a wide variety of alternatives in in­
ternal performance, functional ability, and input/out­
put (I/O). Models of System/360 differ in storage speed, width
(the amount of data obtained in each instruction ac­
cess), register width, and capability of simultaneous
processing. Yet these differences do not affect the logi­
cal appearance of System/360 to the programmer. Several cpu's permit a wide choice in internal per­
formance. The range is such that the ratio of internal
performances between the largest and the smallest
model is approximately 50 for scientific computation
and 15 for commercial processing.
All models of System/360 are upward and downward
program compatible, that is, any program gives identi­
cal results on any model. Compatibility allows for
ease in systems growth, convenience in systems back­
up, and simplicity in education.
The compatibility rule has three limitations.
1. The systems facilities used by a program should
be the same in each case. Thus, the optional CPU fea­
tures and the storage capacity, as well as the quantity,
type, and priority of I/O equipment, should be equiva­
2. The program should be independent of the re­
lation of instruction execution times and of I/O data
rates, access times, and command execution times.
3. The compatibility rule does not apply to detail
functions for which neither frequency of occurrence
nor usefulness of result warrants identical action in
all models. These functions, all explicitly identified in
this manual, are concerned with the handling of in­
valid programs and machine malfunctions.
System Program
Interplay of equipment and program is an essential
consideration in System/360. The system is designed
to operate with a supervisory program that coordi­
nates and executes all I/O instructions, handles excep­
tional conditions, and supervises scheduling and exe­
cution of multiple programs. System/360 provides for
IBM System/360 5
efficient switching from one program to another, as
well as for the relocation of programs in storage. To
the prob1em programmer, the supervisory program
and the equipment are indistinguishable.
System Alerts
The interruption system permits the CPU automatically
to change state as a result of conditions arising out­
side of the system, in 1/0 units, or in the CPU itself.
Interruption switches the CPU from one program to
another by changing not only the instruction address
but all essential machine-status information.
A storage protection feature permits one program to
be preserved when another program erroneously at­
tempts to store information in the area assigned to
the first program. Protection does not cause any loss
of performance. Storage operations initiated from the CPU, as well as those initiated from a channel, are sub­
ject to the protection procedure.
Programs are checked for correct instructions and
data as they are executed. This policing-action identi­
fies and separates program errors and machine errors.
Thus, program errors cannot create machine checks
since each type of error causes a unique interruption.
In addition to an interruption due to machine mal­
function, the information necessary to identify the
error is recorded automatically in a predetermined
storage location. This procedure appreciably reduces
the mean-time to repair a machine fault. Moreover,
operator errors are reduced by minimizing the active
manual controls. To reduce accidental operator errors,
operator consoles are 110 devices and function under
control of the system program.
Multisystem Operation Several models of System/360 can be combined into
one multisystem configuration. Three levels of com­
munication between CPu's are available. Largest in ca­
pacity, and moderately fast in response, is communi­
cations by means of shared 1/0 device, for example,
a disk file. Faster transmission is obtained by direct
connection between the channels of two individual
systems. Finally, storage may be shared on some
models between two cpu's, making information ex­
change possible at storage speeds. These modes of
communication are supplemented by allowing one CPU to be interrupted by another CPU and by making direct
status information available from one CPU to another.
Channels provide the data path and control for 1/0 devices as they communicate with the CPU. In general,
channels operate asynchronously with the CPU and, in
some cases, a single data path is made up of several
sub channels. When this is the case, the single data
path is shared by several low-speed devices, for ex­
ample, card readers, punches, printers, and terminals.
This channel is called a multiplexor channel. Chan­
nels that are not made up of several such subchannels
can operate at higher speed than the multiplexor
channels and are called selector channels. In every
case, the amount of data that comes into the channel
in parallel from an 110 device is a byte. All channels
or subchannels operate the same and respond to the
same 110 instructions and commands.
Each 110 device is connected to one or more chan­
nels by an 110 interface. This 110 interface allows at­
tachment of present and future 110 devices without
altering the instruction set or channel function. Con­
trol units are used where necessary to match the in­
ternal connections of the 110 device to the interface.
Flexibility is enhanced by optional access to a control
unit or dcvice from either of two channels.
Technology System/360 employs solid-logic integrated compo­
nents, which in themselves provide advanced equip­
ment reliability. These components are also faster and
smaller than previous components and lend them­
selves to automated fabrication.
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